In order to work in Sweden, you must have a work permit.
Read more about at Swedish Migration Agency on how to apply for a work permit or for a residence permit as a self-employed person or a visiting researcher.
The quickest and easiest way to apply for a work permit is online. Applications over the internet go directly to the Swedish Migration Board and are given priority.
You may also present your application in person at the Embassy.
If you are interested in working in Sweden, you can also find comprehensive information at the website Working in Sweden.